The present invention relates to a position detecting method, and more particularly to a position detecting method for detecting the position of an object, such as a wafer, in an exposure apparatus used to manufacture various devices including semiconductor chips such as ICs and LSIs, liquid crystal displays, CCDs, and magnetic heads. The present invention is suitable, for example, for an alignment between a reticle and a wafer.
With recent demands for smaller and lower profile electronic apparatuses, finer processing of semiconductor devices to be installed in them has been increasingly demanded. In order to manufacture semiconductor devices according to photolithography, a reduction projection exposure apparatus has conventionally been employed which uses a projection optical system to project a circuit pattern on a mask (or a reticle) onto a wafer, etc. to transfer the circuit pattern.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to the wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength, the better the resolution. Along with recent demands for finer semiconductor devices, shorter wavelengths of ultraviolet light have been promoted from an ultra-high pressure mercury lamp (such as an i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm), an ArF excimer laser (with a wavelength of approximately 193 nm), an F2 laser (with a wavelength of approximately 157 nm), and a synchrotron radiation (“SR”) light.
The projection exposure apparatus needs to execute a highly precise alignment between a reticle and a wafer with finer circuit patterns or higher resolutions. The typical precision necessary for the alignment is about one-third the circuit pattern: For example, the overlay accuracy of 60 nm or smaller is required for a design rule of 0.18 μm for 1-Gbit DRAM circuit patterns. Overlay means an alignment of an entire exposure area.
The high overlay accuracy on a wafer is the vital issue for an exposure apparatus in the current semiconductor industry to improve the performance and manufacture yield or throughput of semiconductor devices. Accordingly, an alignment optical system is used to optically detect the position of an alignment mark formed on a wafer etc., and the wafer is aligned with a reticle, based on the detection result. More specifically, a position of an alignment mark is detected by processing an alignment signal obtained from the alignment mark. Various proposals have been made for signal processing, for example, a folded symmetrical processing that utilizes symmetry of an alignment signal (see, for example, Japanese Patent Application, Publication No. 8-094315).
While a certain alignment repetitively detects positions for all the exposure positions or shots (which is referred to as a “die-by-die system”), a global alignment with a good throughput is usually used. The global alignment measures positional coordinates of plural sample shots on a wafer, statistically processes the measured values, and calculates the shift, magnification and rotational errors of the wafer. After the wafer coordinate system is corrected based on these errors, the stepwise movement to each shot follows. Used more recently is an advanced global alignment (“AGA”) that develops the global alignment and measures a wafer's position by relying on the accuracy of an XY stage that is equipped with a laser interferometer. The AGA calculates the wafer's magnification, rotation and shift amounts, and applies statistical processing, such as the removal of an abnormal value. The abnormal-value removal calculates the average and standard deviation from the measured values of the shots, and eliminates the measured value of a greatly deviated shot.
However, the introduction of special semiconductor manufacture technology, such as a chemical mechanical polishing (“CMP”) process, causes alignment-mark shapes to scatter among wafers and shots, lowering the alignment accuracy disadvantageously. This is because the process condition including a coating, etching and CMP is optimized for fine circuit patterns with CDs between 0.1 μm to 0.15 μm, and is not optimized for alignment marks having large CDs, for example, between 0.6 μm to 4.0 μm, as the difference between the CD of the circuit pattern and that of the alignment mark increases with fine processing of the circuit pattern.
While it is conceivable to fit the alignment mark's CD with the circuit pattern's CD, the lack of resolution of the alignment optical system reduces the signal strength or contrast, and deteriorates the alignment signal's stability. In addition, the alignment optical system for detecting an alignment mark having a CD equivalent to the circuit pattern requires a high NA and a short-wave alignment light source, increasing the cost of an apparatus.
Moreover, a wafer-induced shift (“WIS”) causes a large error in the detection result or makes an alignment signal asymmetrical, and deteriorates the alignment accuracy. That major factor results in an asymmetrical alignment mark and/or resist. An interaction between the WIS and a tool induced shift (“TIS”) results from an exposure apparatus or an alignment optical system, i.e., a TIS-WIS interaction, and also deteriorates the alignment accuracy.
The AGA provides abnormal-value removal to the measured values that include these errors, and measures substitute shots when there are many abnormal shots. However, a measurement of the substitute shot includes an error due to the WIS, and the problem may not be solved completely due to a measurement error amount, although it is small.
An overlay result is confirmed after an exposure, and an overlay error is eliminated through an offset correction. However, the offset correction lowers the throughput, and thus is practically conducted for each lot or for each process. Therefore, the problem still remains. It is possible to identify a shot that causes an error based on the overlay result, but it takes a long time.